JP7553265B2 - Porous ceramic laminate and manufacturing method thereof - Google Patents

Description

The present invention relates to a porous ceramic laminate and a method for manufacturing the same.

Porous ceramics are used in a variety of fields as membranes with functions such as separating, concentrating, and filtering gases, liquids, and other fluids, including microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes, ion exchange membranes, and gas separation membranes.

For example, Patent Document 1 discloses a ceramic porous asymmetric membrane consisting of an inorganic support and one or more inorganic coating layers covering the support, in which at least one of the one or more inorganic coating layers covering the support contains a thin-layered inorganic porous body. In Patent Document 1, a slurry membrane containing alumina with an average particle size of 3 or 5 μm is formed on a porous support having an average pore size of 11.0 or 14.2 μm, and the porous layer is formed on the porous support by firing. Patent Document 2 discloses an asymmetric membrane consisting of a support layer and a porous layer formed on the inner or outer surface of the support layer. In Patent Document 2, a support layer with a pore size of 1 to 2 μm is created using alumina particles with an average particle size of 5 μm or particles of other inorganic substances, and a porous layer is formed on the surface of the support layer using a liquid in which alumina fine powder with an average particle size of 0.5 μm is dispersed.

Japanese Patent Application Publication No. 11-292653 Japanese Unexamined Patent Publication No. 186908/1986

When using porous ceramics to separate fluids, it is important that the fluid flows easily through the porous ceramics while still fully performing the function of separation, i.e., that the pressure loss of the fluid is small. However, it is considered difficult to reduce the pressure loss of the fluid in Patent Documents 1 and 2.

Therefore, the present invention aims to provide a porous ceramic laminate that can reduce fluid pressure loss.

The present invention which achieves the above object is as follows.
[1] A porous ceramic laminate having a first porous layer and a second porous layer laminated on the first porous layer,
the second porous layer has a portion laminated on and in contact with the first porous layer and a portion laminated on the first porous layer with air interposed therebetween;
A porous ceramic laminate, wherein the coefficient of variation CV(t _b ) of the thickness of the second porous layer is 0.35 or less.
[2] The porous ceramic laminate according to [1], wherein the average pore diameter D _a of the first porous layer is 0.1 μm or more and 600 μm or less.
[3] A porous ceramic laminate according to [1] or [2], wherein the value (σ(t _b )/D _a ) obtained by dividing the standard deviation σ(t _b ) of the thickness of the second porous layer by the average pore diameter D _a of the first porous layer is 0.8 or less.
[4] A porous ceramic laminate according to any one of [1] to [3], wherein the value (AVE(t _b )/D _a ) obtained by dividing the average thickness AVE(t _b ) of the second porous layer by the average pore diameter D _a of the first porous layer is 5 or less.
[5] The porous ceramic laminate according to any one of [1] to [4], wherein the porosity at the interface between the first porous layer and the second porous layer is 29.0% or more.
[6] The porous ceramic laminate according to any one of [1] to [5], wherein the ratio (D _a /D _b ) of the average pore diameter D _a of the first porous layer to the average pore diameter D _b of the second porous layer is 10 or more.
[7] A method for producing a porous ceramic laminate according to any one of [1] to [6],
A step of applying a water repellent and/or an oil repellent to at least one surface of the first porous layer,
The method for producing a porous ceramic laminate, wherein the water repellent and/or oil repellent is applied so as to satisfy the following conditions (1) and (2):
(1) A water-repellent and/or oil-repellent is applied to at least one surface of the first porous layer, and after drying, when air is caused to flow at a flow rate of 1.0 ^m3 /h from the water-repellent and/or oil-repellent coated surface of the first porous layer (or from either one of the coated surfaces if coated on both surfaces), air flows at a flow rate of at least 0.8 ^m3 /h from the surface opposite to the surface through which the air is flowing.
(2) When a water-repellent and/or oil-repellent is applied to at least one surface of the first porous layer and dried, and then air is caused to flow at a flow rate of 1.0 ^m3 /h from the water-repellent and/or oil-repellent coated surface of the first porous layer (or from either one of the coated surfaces if coated on both surfaces), the weight loss rate before and after the air flow test is within 0.1%.

The porous ceramic laminate of the present invention can reduce pressure loss of fluids such as gases and liquids.

11 is an analysis image obtained when the film thickness of a second porous layer is measured in Example 2. FIG. 2 is a schematic diagram showing a method for measuring porosity.

The porous ceramic laminate of the present invention is a porous ceramic laminate having a first porous layer and a second porous layer laminated on the first porous layer, wherein the second porous layer has a portion laminated in contact with the first porous layer and a portion laminated on the first porous layer with air between them, and is characterized in that the coefficient of variation CV(t _b ) of the film thickness of the second porous layer is 0.35 or less.

A part of the second porous layer is laminated on top of the first porous layer in contact with it, and the other part of the second porous layer is laminated on the first porous layer via air. The fact that the second porous layer has a portion laminated on the first porous layer via air is an important requirement for reducing pressure loss of the fluid.

Furthermore, in the porous ceramic laminate, the coefficient of variation CV(t _b ) of the thickness of the second porous layer is uniformly formed to be 0.35 or less, thereby reducing the pressure loss of the fluid flowing through the porous ceramic laminate. The coefficient of variation CV(t _b ) of the thickness t _b of the second porous layer is preferably 0.32 or less, more preferably 0.25 or less, and even more preferably 0.20 or less. The lower limit of the coefficient of variation is not particularly limited, but may be, for example, 0.05 or 0.10. The thickness t _b can be measured as follows, and the coefficient of variation CV(t _b ) of the thickness can be obtained by dividing the standard deviation σ(t _b ) of the thickness by the average thickness AVE(t _b ). First, an image of a cross section parallel to the lamination direction of the first porous layer and the second porous layer is obtained. In the image, the total length of the surface of the second porous layer on the side of the first porous layer is 100 times or more the average pore diameter of the first porous layer, and only the second porous layer is extracted by image analysis. The obtained image is divided at regular intervals into 190 or more regions, the area of each region is calculated, and the value divided by the length of the divided interval is regarded as the film thickness of the second porous layer in each region, and the average value, standard deviation, and coefficient of variation of the film thickness for all the measured regions are calculated. However, the length of the divided interval is 1.8 times or less the average pore diameter of the first porous layer.

As long as the coefficient of variation CV(t _b ) of the thickness is 0.35 or less, the average value AVE(t _b ) of the thickness of the second porous layer is not limited, but the average value AVE(t _b ) is, for example, 0.3 μm or more, more preferably 1.0 μm or more, even more preferably 5.0 μm or more, even more preferably 10 μm or more, and for example, 3000 μm or less, more preferably 500 μm or less, even more preferably 100 μm or less, even more preferably 75 μm or less, and particularly preferably 50 μm or less. The average value AVE(t _b ) may be in the range of 15 μm or more and 25 μm or less.

It is also preferable to adjust the value (σ(t _b )/D _a ) obtained by dividing the standard deviation σ(t _b ) of the thickness of the second porous layer by the average pore diameter D _a of the first porous layer. σ(t _b )/D _a is preferably 0.8 or less, more preferably 0.6 or less, even more preferably 0.5 or less, and even more preferably 0.4 or less. The lower limit of σ(t _b )/D _a is not particularly limited, but may be about 0.2. σ(t _b )/D _a represents the standard deviation of the thickness of the second porous layer relative to the average pore diameter of the first porous layer, and the smaller this value is, the more uniformly the second porous layer can be coated. From the viewpoint of reducing pressure loss, it is preferable that σ(t _b )/D _a is smaller.

Furthermore, it is also preferable that the value (AVE(t _b )/D _a ) obtained by dividing the average thickness AVE(t _b ) of the second porous layer by the average pore diameter _Da of the first porous layer is 5 or less, preferably 4 or less, and more preferably 3 or less. The lower limit is not particularly limited, but may be, for example, about 0.5.

From the viewpoint of reducing the pressure loss of the fluid, it is also preferable to set the porosity at the interface between the first porous layer and the second porous layer to a predetermined value or more. The porosity can be obtained as follows. First, an image of a cross section parallel to the lamination direction of the first porous layer and the second porous layer is obtained. In the image, an image is obtained so that the total length of the first porous layer side surface of the second porous layer is 250 times or more the average pore diameter of the first porous layer, and only the second porous layer is extracted by image analysis. For the region below the extracted second porous layer (from the first porous layer side surface of the second porous layer to the first porous layer side), a region twice the pore diameter of the first porous layer is set by image analysis along the second porous layer from the boundary of the second porous layer on the first porous layer side, and set as the analysis region. The void portion in this analysis region is extracted by image analysis, and the area of the void portion is divided by the area of the analysis region and multiplied by 100 to obtain the porosity.

The porosity is preferably 29.0% or more, more preferably 30% or more, and even more preferably 33% or more. The upper limit of the porosity depends on the porosity of the first porous layer, but is, for example, 55%.

In addition, the ratio (D _a /D _b ) of the average pore diameter D _a of the first porous layer to the average pore diameter D _b of the second porous layer is preferably 10 or more. In this way, by increasing the difference between Da and Db, the pressure loss of the fluid passing through the porous ceramic laminate can be further reduced, which is preferable. _{D a} /D _b is preferably 10 or more and 200 or less, more preferably 10 or more and 150 or less, even more preferably 15 or more and 100 or less, and even more preferably 20 or more (particularly 50 or more) and 65 or less. In addition, in the above-mentioned Patent Document 1, the porous layer is formed on the porous support using a slurry containing alumina having an average particle diameter of 3 or 5 μm, and considering the particle diameter of the alumina, the average pore diameter of the porous layer is considered to be about 1 μm, and as described above, the average pore diameter of the porous support is 11.0 or 14.2 μm. Therefore, the ratio of the average pore diameter of the support to the average pore diameter of the porous layer in the above-mentioned Patent Document 1 is considered to be about 14 at most. In addition, in Patent Document 2, the support layer has an interconnected pore size of 1 to 2 μm, and the porous layer has a pore size of 0.2 μm, so the ratio of the interconnected pore size of the support layer to the pore size of the porous layer is 10 at most.

The average pore diameter D _a of the first porous layer is, for example, 0.1 μm or more, preferably 1.0 μm or more, more preferably 1.5 μm or more, even more preferably 5 μm or more, even more preferably 9 μm or more, and may be 600 μm or less, preferably 300 μm or less, and more preferably 60 μm or less. _{D a} may be in the range of 12 μm or more and 18 μm. The average pore diameter D b of the second porous layer is, for example, 0.01 μm or more and 10 μm or less, preferably 0.05 μm or more and 5 μm or less, more preferably 0.15 μm or more and 1 μm or less, and even more preferably 0.20 μm or more and 1 μm or less.

The average thickness of the first porous layer is, for example, 500 μm or more and 5000 μm or less, preferably 1000 μm or more and 2800 μm or less, and more preferably 1400 μm or more and 1900 μm or less.

The second porous layer may be laminated on at least one side of the first porous layer, and is preferably laminated on only one side of the first porous layer. The shape of the porous ceramic laminate of the present invention is not particularly limited, and may be planar, cylindrical, or honeycomb, and is preferably cylindrical. When the porous ceramic laminate of the present invention is cylindrical, the second porous layer may be laminated on either the outer peripheral surface or the inner peripheral surface of the first porous layer, and is preferably laminated only on the outer peripheral surface or the inner peripheral surface of the first porous layer, and it is more preferable that the second porous layer is laminated only on the outer peripheral surface of the first porous layer.

The materials of the first and second porous layers are not particularly limited, but it is preferable that both contain a metal oxide. In the present invention, the term "metal" is used to mean a metalloid such as Si or Ge. The first porous layer preferably contains, as the metal oxide, a metal oxide A and a metal oxide B having a melting point higher than that of the metal oxide A, and the second porous layer preferably contains, as the metal oxide, a metal oxide C having a melting point higher than that of the metal oxide A. The metal oxide B and the metal oxide C may be the same or different.

The metal oxide A preferably has a melting point of 95° C. or higher and 1600° C. or lower. Specific _examples of the metal oxide A include _B2O3 , _SiO2 , _GeO2 , _Al2O3 , _V2O5 , _As2O5 , _Sb2O5 , _{ZrO2 , TiO2} , _ZnO , PbO, _ThO2 , _BeO , CdO, _Ta2O5 , _Nb2O5 , _{WO3 , ScO2 , La2O3} , _Y2O3 , _SnO2 , _Ga2O3 , _In2O3 , _PbO2 , _MgO , _Li2O , BaO, CaO _{, SrO , Na2O} , _K2O , Rb2O, HgO, _Cs2O , and _{Ag . 2O} , _TeO2 , _Tl2O , etc., preferably _SiO2 . That is, the metal oxide A may be one having at least one of the above oxides as a constituent, and is preferably a glass having at least one of the above oxides as a constituent, and is preferably a glass containing an oxide of silicon (particularly _SiO2 ), such as quartz glass, borosilicate glass, or alumina silicate glass. The metal oxide B and the metal oxide C may be the same or different, and each preferably has a melting point of 2000°C or more and 2800°C or less. Specific examples of the metal oxide B and the metal oxide _C include _{Al2O3 , ZrO2} , _MgO , _Cr2O3 , _{Y2O3 , etc.} , and is preferably _Al2O3 .

In addition to the first and second porous layers, the porous ceramic laminate may further include one or more other porous layers laminated on the second porous layer. The other porous layers preferably have a smaller average pore diameter than the second porous layer, and when there are two or more other porous layers, it is preferable that the average pore diameter of the other porous layer closest to the second porous layer is the largest, and the average pore diameter decreases as the layers are laminated.

The porous ceramic laminate of the present invention can reduce the pressure loss of a fluid. The porous ceramic laminate can have a permeance evaluated in the examples described below of 5.0×10 ⁻⁵ mol/(m ² ·sec·Pa) or more, preferably 1.0×10 ⁻⁴ mol/(m ² ·sec·Pa) or more, and more preferably 1.5×10 ⁻⁴ mol/(m ² ·sec·Pa) or more. The upper limit of the permeance is not particularly limited, but may be, for example, 1.5×10 ⁻³ mol/(m ² ·sec·Pa) or 5.0×10 ⁻⁴ mol/(m ² ·sec·Pa).

Next, a method for manufacturing the porous ceramic laminate will be described. Normally, when attempting to laminate another layer on a porous layer, the components of the upper layer (second porous layer) are incorporated into the pores of the porous layer (first porous layer) that serves as the base material, making it difficult to form the second porous layer in a layered form, and furthermore, it is difficult to form a second porous layer with a highly uniform film thickness.

The method for manufacturing the porous ceramic laminate includes a step of applying a water repellent and/or oil repellent to at least one surface of the first porous layer. By applying the water repellent and/or oil repellent, the components of the second porous layer to be applied thereafter are not taken into the pores of the first porous layer, so that a second porous layer having a laminar shape and a highly uniform thickness can be formed on the first porous layer. The above-mentioned "surface" includes the pores present on the surface of the porous body, and the water repellent and/or oil repellent is applied to at least one surface of the porous body including the pores. Here, the water repellent or oil repellent is applied so that the following conditions (1) and (2) are satisfied.

(1) A water-repellent and/or oil-repellent is applied to at least one surface of the first porous layer, and after drying, when air is caused to flow at a flow rate of 1.0 ^m3 /h from the water-repellent and/or oil-repellent coated surface of the first porous layer (or from either one of the coated surfaces if coated on both surfaces), air flows at a flow rate of at least 0.8 ^m3 /h from the surface opposite to the surface through which the air is flowing.
(2) When a water-repellent and/or oil-repellent is applied to at least one surface of the first porous layer body and dried, and then air is passed through the water-repellent and/or oil-repellent-coated surface of the first porous layer (or through either one of the coated surfaces if coated on both surfaces) at a flow rate of 1.0 ^m3 /h, the weight loss rate before and after the air passage test is within 0.1%.

In other words, it is important that the water repellent and/or oil repellent is applied so that when a predetermined flow rate of air is passed through the first porous layer after the water repellent and/or oil repellent is applied, ventilation is ensured and the water repellent and/or oil repellent does not separate from the first porous layer.

Regarding the above condition (1), when the water repellent and/or oil repellent is applied to one side of the first porous layer, air is allowed to flow from the side to which the water repellent and/or oil repellent is applied and the air flow rate on the opposite side is measured. When the water repellent and/or oil repellent are applied to both sides, air is allowed to flow from one of the applied sides and the air flow rate on the opposite side is measured. The time for which the air is allowed to flow under the above condition (1) is 3 minutes, and the temperature of the air being allowed to flow is 10 to 35°C.

Regarding the above condition (2), when the water repellent and/or oil repellent is applied to one side of the first porous layer, air is allowed to flow from the surface to which the water repellent and/or oil repellent is applied, and when the water repellent and/or oil repellent are applied to both sides, air is allowed to flow from either one of the surfaces to measure the weight reduction rate. The weight reduction rate is the ratio (percentage) obtained by subtracting the weight of the first porous layer after air has been allowed to flow from the weight of the first porous layer before air is allowed to flow, by the weight of the first porous layer before air is allowed to flow.

The preferred method for applying the water and/or oil repellent is dip coating, and after application of the water and/or oil repellent, drying is usually performed. Drying is preferably performed at about 20 to 50°C for about 1 minute to 24 hours. It is also preferred that the water and/or oil repellent remains after application and drying. As long as the above conditions (1) and (2) are satisfied, the composition of the water and oil repellent is not limited, but for example, fluorine-based water and oil repellents and polysiloxane-based water and oil repellents can be used. Examples of fluorine-based water and oil repellents include those having a perfluoroalkyl group and those having a perfluoropolyether structure.

It is preferable that the manufacturing method further includes a step of applying a slurry containing the metal oxide contained in the second porous layer, a solvent, and a thickener to the surface of the first porous layer to which the water repellent or the oil repellent has been applied, and a step of heat treating the first porous layer to which the slurry has been applied.

Examples of the solvent include water and organic solvents. In a preferred embodiment, the metal oxide contained in the second porous layer more preferably contains the metal oxide C. The concentration of the metal oxide in the slurry is, for example, 2 mass% or more and 15 mass% or less, preferably 4 mass% or more and 13 mass% or less. When the second porous layer contains multiple types of metal oxides, the concentration means the total concentration of the multiple types of metal oxides. The average particle size of the metal oxide contained in the second porous layer is, for example, 0.1 μm or more and 50 μm or less, 0.3 μm or more and 30 μm or less, 0.3 μm or more and 20 μm or less, 0.5 μm or more and 10 μm or less, or 0.5 μm or more and 5 μm or less.

Examples of the thickener include methyl cellulose, hydroxyethyl methyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, polyalkylene oxide, polyvinyl alcohol, sodium polyacrylate, polyvinylpyrrolidone, polyacrylamide, polydimethylaminoethyl methacrylate, etc. The concentration of the thickener in the slurry is, for example, 0.5% by mass or more and 5% by mass or less, and preferably 1% by mass or more and 3% by mass or less.

The method for applying the slurry to the surface of the first porous layer to which the water repellent or oil repellent has been applied is not particularly limited, and examples of the method include dip coating, spray coating, roll coating, bar coating, spin coating, slit coating, brush coating, etc.

The temperature of the heat treatment is, for example, 95°C or higher, preferably 265°C or higher, more preferably 500°C or higher, and even more preferably 1000°C or higher. The temperature of the heat treatment is, for example, 1600°C or lower, preferably 1400°C or lower. The temperature of the heat treatment is preferably equal to or higher than the softening temperature of the metal oxide A, and specifically, the temperature of the heat treatment is preferably 1000°C or higher and 1500°C or lower, and more preferably 1100°C or higher and 1400°C or lower. The holding time at the heat treatment temperature is, for example, 30 minutes or more and 10 hours or less, preferably 1 hour or more and 8 hours or less, and more preferably 3 hours or more and 7 hours or less.

The porous ceramic laminate can be used as a precision filtration membrane. By further laminating a functional membrane on the second porous layer in the porous ceramic laminate, the porous ceramic laminate of the present invention on which the functional membrane is laminated can also be used as a substrate for membranes such as ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes, ion exchange membranes, and gas separation membranes.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can of course be modified as appropriate within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

The ceramic laminates obtained in the following examples and comparative examples were evaluated by the following methods.

(1) Measurement of pore size After drying the porous ceramic laminate sample at 120 ° C. for 4 hours, the pore size was measured by mercury intrusion using Autopore IV9520 (manufactured by Micromeritics). When the first porous layer before laminating the second porous layer and the porous ceramic laminate sample of the following example were measured, in the log differential pore volume distribution with the pore size on the horizontal axis, one peak was observed in the first porous layer before laminating the second porous layer, and two peaks were observed in the porous ceramic laminate of the following example. The peak position observed in the first porous layer before laminating the second porous layer was taken as D _a . Of the two peaks observed in the porous ceramic laminate of the following example, the peak position on the lower pore size side was taken as D _b .

(2) Measurement of the coefficient of variation of the thickness of the second porous layer A cylindrical porous ceramic laminate was embedded in resin, polished, and a cross section perpendicular to the axial direction (i.e., a cross section parallel to the lamination direction of the first and second porous layers) was taken and observed with a scanning electron microscope (SEM). An image was obtained so that the total length of the first porous layer side surface of the second porous layer was 1.5 mm or more in the circumferential direction, and only the second porous layer was extracted by image analysis. The obtained image was divided at an 8 μm pitch to create 190 or more regions, and the area of each was calculated and divided by 8 μm to obtain the thickness of each region. The average value and standard deviation were calculated from the obtained thickness, and the standard deviation was divided by the average value to obtain the coefficient of variation. Furthermore, the average thickness AVE (t _b ) and standard deviation σ (t _b ) of the second porous layer were each divided by the pore diameter D _a of the first porous layer. Fig. 1 shows a part of the image analysis when the thickness of the second porous layer was measured in Example 2. In Fig. 1, 1 is the first porous layer, 2 is the second porous layer, and 3 is the measurement interval (8 µm in Fig. 1).

(3) Measurement of porosity A cylindrical porous ceramic laminate was embedded in resin, polished, and a cross section perpendicular to the axial direction was taken and observed with a scanning electron microscope (SEM, Scanning Electron Microscope). An image was acquired so that the total length of the first porous layer side surface of the second porous layer was 4.0 mm or more in the circumferential direction, and only the second porous layer was extracted by image analysis. For the extracted lower region of the second porous layer, a region twice the pore diameter of the first porous layer was set along the second porous layer from the first porous layer side surface of the second porous layer (i.e., from the boundary of the first porous layer side of the second porous layer) by image analysis, and was set as the analysis region. The void portion in this analysis region was extracted by image analysis, and the area of the void portion was divided by the area of the analysis region and multiplied by 100 to obtain the porosity. FIG. 2 shows a schematic diagram of a method for measuring the porosity. In FIG. 2, 1 is a first porous layer, 2 is a second porous layer, 2' is a boundary of the second porous layer on the side of the first porous layer, and 4 is a gap.

(4) Measurement of permeance Air was made to flow from the outside of a cylindrical porous ceramic laminate sample at a constant flow rate of 1.0 ^m3 /h, and the air was allowed to permeate from the inside of the sample. The pressure difference before and after the air permeated the sample was measured. Using the measurement results, the permeance was calculated according to the following formula.

Example 1
As the first porous layer, alumina substrate A-16 manufactured by Hagi Glass Co., Ltd., containing alumina and SiO _2, was used. The shape of the substrate A-16 was a cylinder with an inner diameter of 8.6 mm, an outer diameter of 11.5 mm, and a length of 5 cm. The substrate A-16 was dipped in a water repellent SH Conc (containing fluororesin and petroleum-based hydrocarbons) manufactured by Columbus Co., Ltd., and dried at room temperature for 12 hours or more. Next, alumina powder AKP-3000 manufactured by Sumitomo Chemical Co., Ltd. and hydroxypropyl methylcellulose 65SH-30000 manufactured by Shin-Etsu Chemical Co., Ltd. as a thickener were mixed in water at concentrations of 5 wt% and 1.5 wt%, respectively, to prepare a slurry. The average particle size of the alumina powder AKP-3000 was 0.7 μm. The upper and lower ends of the substrate A-16 were sealed so that the slurry would not get into the inner peripheral surface of the substrate A-16, and the substrate A-16 was dip-coated with the slurry. Thereafter, the substrate A-16 with the slurry applied to the outer peripheral surface was heat-treated at 1200°C for 3 hours. After the water repellent SH Conc was applied and dried, it was confirmed that the requirements of the above conditions (1) and (2) were met.

Comparative Example 1
A sample was obtained in the same manner as in Example 1, except that the water repellent SH Conc manufactured by Columbus Co., Ltd. was not dipped.

Example 2
As the first porous layer, alumina substrate A-12 manufactured by Hagi Glass Co., Ltd., containing alumina and SiO _2, was used. The shape of the substrate A-12 was a cylinder with an inner diameter of 7.0 mm, an outer diameter of 9.5 mm, and a length of 10 cm. The substrate A-12 was dipped in a water repellent SH Conc manufactured by Columbus Co., Ltd., and dried at room temperature for 12 hours or more. Next, alumina powder AKP-3000 manufactured by Sumitomo Chemical Co., Ltd. and hydroxypropyl methylcellulose 65SH-30000 manufactured by Shin-Etsu Chemical Co., Ltd. as a thickener were mixed in water at concentrations of 5 wt% and 1.5 wt%, respectively, to prepare a slurry. The average particle size of the alumina powder AKP-3000 was 0.7 μm. The upper and lower ends of the substrate A-12 were sealed so that the slurry would not enter the inner peripheral surface of the substrate A-12, and the substrate A-12 was dip-coated with the slurry. Thereafter, the substrate A-12 with the slurry applied to the outer peripheral surface was heat-treated at 1200° C. for 3 hours. The application of the slurry and the heat treatment were repeated once more to obtain a sample. It was confirmed that the above-mentioned conditions (1) and (2) were satisfied after the water repellent SH Conc was applied and dried.

Comparative Example 2
A sample was obtained in the same manner as in Example 2, except that the water repellent SH Conc manufactured by Columbus Co., Ltd. was not dipped.

The results of measurements taken in accordance with (1) to (4) above for the Examples and Comparative Examples are shown in Table 1.

Comparing Example 1 and Comparative Example 1 in Table 1, Example 1, in which the coefficient of variation of the film thickness of the second porous layer was 0.35 or less, achieved good permeance, whereas Comparative Example 1, in which the coefficient of variation exceeded 0.35, achieved poorer permeance than Example 1. In Comparative Example 2, in which a laminate was prepared in the same manner as Example 2 except that no water repellent agent was used, the second porous layer could not be formed, whereas Example 2 was able to form a second porous layer with a coefficient of variation of the film thickness of 0.35 or less, achieving good permeance.

Claims (3)

A porous ceramic laminate having a first porous layer and a second porous layer laminated on the first porous layer,
the second porous layer has a portion laminated on and in contact with the first porous layer and a portion laminated on the first porous layer with air interposed therebetween;
the coefficient of variation CV(t _b ) of the thickness of the second porous layer is 0.05 or more and 0.35 or less ;
a value (σ(t _b )/D _a ) obtained by dividing a standard deviation σ(t _b ) of a thickness of the second porous layer by an average pore diameter D _a of the first porous layer is 0.2 or more and 0.8 or less;
a value (AVE(t _b )/D _a ) obtained by dividing an average thickness AVE(t _b ) of the second porous layer by an average pore diameter D _a of the first porous layer is 0.5 or more and 5 or less,
a porosity at an interface between the first porous layer and the second porous layer is 29.0% or more and 55% or less;
a ratio (D _a /D _b ) of an average pore diameter D _a of the first porous layer to an average pore diameter D _b of the second porous layer is 10 or more and 200 or less;
A porous ceramic laminate, wherein the coefficient of variation CV(t _b ) and the standard deviation σ(t _b ) of the film thickness are values that can be determined as follows :
(i) An image of a cross section parallel to the lamination direction of the first porous layer and the second porous layer is obtained.
(ii) The image is acquired so that the total length of the surface of the second porous layer facing the first porous layer is 100 times or more the average pore diameter of the first porous layer.
(iii) Extract only the second porous layer by image analysis, divide the obtained image at regular intervals into 190 or more regions, calculate the area of each region, and divide the area by the length of each divided region to obtain the film thickness of the second porous layer in each region, where the length of each divided region is 1.8 times or less the average pore diameter of the first porous layer.
(iv) The average film thickness AVE(t _b ) and the standard deviation σ(t _b ) of the film thickness for the entire measured region are calculated .
(v) The standard deviation σ(t _b ) of the film thickness is divided by the average film thickness AVE(t _b ) to obtain the coefficient of variation CV(t _b ) of the film thickness . 2. The porous ceramic laminate according to claim 1, wherein the first porous layer has an average pore diameter D _a of 0.1 μm or more and 600 μm or less. A method for producing the porous ceramic laminate according to claim 1 or 2 , comprising the steps of:
applying a water- and oil-repellent agent to at least one surface of the first porous layer;
The water- and oil-repellent agent is applied so as to satisfy the following conditions (1) and (2):
(1) A water- and oil -repellent agent is applied to at least one surface of the first porous layer, and after drying, when air is caused to flow at a flow rate of 1.0 ^m3 /h from the water- and oil-repellent agent-coated surface of the first porous layer (or from either surface if coated on both surfaces), air flows at a flow rate of at least 0.8 ^m3 /h from the surface opposite to the surface through which the air is flowing.
(2) A water- and oil -repellent agent is applied to at least one surface of the first porous layer, and after drying, air is passed through the water- and oil-repellent coated surface of the first porous layer (or through either one of the coated surfaces if coated on both surfaces) at a flow rate of 1.0 ^m3 /h; the weight loss rate before and after the air passage test is within 0.1%.